8.2 Neonatal Sepsis & Infectious Pathogens

8.2 Neonatal Sepsis & Infectious Pathogens

Neonatal sepsis is a clinical syndrome characterized by systemic signs of infection accompanied by bacteremia or meningitis within the first 28 days of life. Due to an immature immune system, neonates—especially preterm and very low birth weight (VLBW) infants—are highly susceptible to rapid pathogen dissemination, septic shock, and death. The neonatal immune response is limited by a small neutrophil storage pool, impaired leukocyte chemotaxis, low levels of complement proteins, and a lack of prior antigen exposure. Immunoglobulin G (IgG) is actively transferred across the placenta only after 32 weeks gestation, leaving extremely preterm infants immunologically vulnerable. Neonatal sepsis is classified into early-onset sepsis (EOS) and late-onset sepsis (LOS).

---

Early-Onset Sepsis (EOS)

Early-onset sepsis occurs within the first 72 hours of life. It is acquired vertically, meaning the infant is exposed to pathogens in the maternal genitourinary tract before or during delivery.

Maternal and Intrapartum Risk Factors

• Group B Streptococcus (GBS) Colonization: Maternal GBS colonization of the vagina or rectum (identified via screening at 36 0/7 to 37 6/7 weeks).
• Prolonged Rupture of Membranes (ROM): Rupture of amniotic membranes \ge18 hours prior to delivery, which allows ascending bacterial migration.
• Maternal Intrapartum Fever: Temperature \ge38.0°C (100.4°F) during labor.
• Chorioamnionitis: Defined clinically as maternal fever plus maternal tachycardia, fetal tachycardia, uterine tenderness, or foul-smelling amniotic fluid (often classified as "triple I": intraamniotic infection or inflammation).
• Maternal GBS Bacteriuria: GBS detected in maternal urine at any point during the current pregnancy.
• Previous Infant with GBS Disease: Represents a high-risk history.
• Prematurity: Gestational age <37 weeks.

Pathogens Associated with EOS

• Group B Streptococcus (GBS, Streptococcus agalactiae): The most common pathogen in term infants.
• Escherichia coli: The most common pathogen in preterm and VLBW infants, often associated with a higher mortality rate than GBS.
• Listeria monocytogenes: A Gram-positive rod acquired via maternal ingestion of contaminated foods (e.g., unpasteurized dairy, deli meats). It can cause granulomatosis infantiseptica.
• Haemophilus influenzae: Gram-negative coccobacillus, increasingly common in preterm infants.

---

Late-Onset Sepsis (LOS)

Late-onset sepsis occurs after 72 hours of life (typically between 4 days and 28 days, or up to discharge in preterm infants). It is acquired horizontally from the hospital environment (nosocomial) or the community.

Neonatal Risk Factors

• Prematurity and VLBW: Immature skin and mucosal barriers.
• Indwelling Central Venous Catheters: Umbilical venous catheters (UVCs) or peripherally inserted central catheters (PICCs) provide a direct portal of entry for pathogens.
• Parenteral Nutrition (TPN): Intravenous lipid emulsions promote bacterial and fungal growth.
• Mechanical Ventilation: Bypasses upper airway defenses, increasing the risk of ventilator-associated pneumonia.
• Delayed Enteral Feeding: Leads to mucosal atrophy and bacterial translocation across the gut wall.
• Broad-Spectrum Antibiotics: Disrupts the neonatal microbiome, selecting for resistant pathogens.

Pathogens Associated with LOS

• Coagulase-Negative Staphylococci (CoNS, Staphylococcus epidermidis): The most common cause of nosocomial sepsis. These bacteria produce a biofilm (slime layer) that allows them to adhere to central catheter plastic, protecting them from antibiotics.
• Staphylococcus aureus: Gram-positive cocci (both methicillin-susceptible [MSSA] and methicillin-resistant [MRSA]).
• Gram-Negative Bacilli: Klebsiella pneumoniae, Pseudomonas aeruginosa, Serratia marcescens, and Enterobacter species. These pathogens can cause rapid clinical deterioration and endotoxic shock.
• Fungal Pathogens: Candida albicans and Candida parapsilosis. Risk factors include extremely low birth weight, prolonged use of third-generation cephalosporins or carbapenems, and central lines.

---

Clinical Manifestations of Sepsis

The clinical presentation of neonatal sepsis is famously subtle and non-specific. The RNC-NIC nurse must recognize early, multi-systemic signs:

Systemic Manifestations

---

Neonatal Meningitis

Neonatal meningitis occurs in up to 15% of infants with early-onset sepsis and up to 20% of those with late-onset sepsis.

• Clinical Signs: Bulging fontanelle, significant lethargy or irritability, high-pitched cry, seizures, and temperature instability. Classic signs of meningeal irritation (e.g., nuchal rigidity, Kernig's, Brudzinski's signs) are rarely present in neonates.
• Diagnosis: Standard of care is a Lumbar Puncture (LP) for cerebrospinal fluid (CSF) analysis, unless the infant is too unstable to tolerate the positioning.
  • CSF WBC: \ge20–30 cells/mm³ (predominantly PMNs).
  • CSF Protein: Elevated (>150 mg/dL in term; >170 mg/dL in preterm).
  • CSF Glucose: Decreased (<30–50% of a simultaneous blood glucose level, or absolute glucose <30 mg/dL).
  • Gram Stain and Culture: Positive for the offending pathogen.

---

Laboratory Diagnostic Criteria

• Blood Culture: The gold standard. A minimum of 0.5 to 1.0 mL of blood must be inoculated into a pediatric bottle. Drawing from a central line is associated with a high rate of contamination; a peripheral sterile stick is preferred.
• Complete Blood Count (CBC) with Differential:
  • Absolute Neutrophil Count (ANC): Neutropenia (ANC <1,500/mm³) is a much stronger and more specific predictor of sepsis than leukocytosis (WBC >25,000/mm³).
  • Immature-to-Total (I:T) Ratio: The ratio of immature neutrophils (bands, metamyelocytes, myelocytes) to total neutrophils (segmented plus immature). An I:T ratio \ge 0.2 is highly suggestive of sepsis.
  • Thrombocytopenia: Platelets <100,000/mm³ is a late, non-specific sign of sepsis, often associated with Gram-negative organisms, fungal infections, or disseminated intravascular coagulation (DIC).
• C-Reactive Protein (CRP): An acute-phase reactant synthesized by the liver in response to tissue injury or infection. A single normal CRP at the onset of symptoms does not rule out sepsis. However, serial CRPs (e.g., at 0, 12, and 24 hours) have a high negative predictive value (99%), allowing safe discontinuation of antibiotics if they remain normal.

---

Pharmacological Treatment & Antibiotic Choices

Empiric antibiotic therapy must be initiated immediately after blood and CSF cultures are obtained.

Early-Onset Sepsis Empiric Therapy

• Antibiotic Regimen: Ampicillin plus Gentamicin (or another aminoglycoside).
  • Ampicillin: Provides coverage for GBS, Listeria monocytogenes, and susceptible Gram-positive cocci.
  • Gentamicin: Provides synergistic Gram-positive coverage and targets Gram-negative rods like E. coli.
  • Pharmacokinetics: Gentamicin is cleared renally. Dosing intervals must be adjusted based on gestational age and postnatal age (e.g., every 36–48 hours for extremely preterm infants vs. every 24 hours for term infants) to prevent nephrotoxicity and ototoxicity. Serum trough levels should be monitored (target trough <1.5 mcg/mL).

Late-Onset Sepsis Empiric Therapy

• Antibiotic Regimen: Vancomycin plus Gentamicin (or Amikacin). If resistant Gram-negative bacilli are suspected, Cefepime or Piperacillin-Tazobactam may be added.
  • Vancomycin: Targets CoNS and MRSA. Monitor serum trough levels (target trough 10–15 mcg/mL, or higher for meningitis).

Meningitis Suspected or Confirmed

• Antibiotic Regimen: Ampicillin plus Cefotaxime (or Ceftazidime).
  • Cefotaxime: A third-generation cephalosporin that achieves excellent cerebrospinal fluid (CSF) penetration and has no renal toxicity.
  • Ceftriaxone Contraindication: Ceftriaxone is contraindicated in neonates. It competes with bilirubin for albumin-binding sites, increasing the risk of kernicterus. Additionally, Ceftriaxone can precipitate with intravenous calcium, forming a fatal crystalline deposit in the lungs and kidneys.

---

Exam Traps

• Urine Culture in EOS: A urine culture is not recommended as part of an early-onset sepsis workup (first 72 hours) because urinary tract infections are exceedingly rare in this window. However, urine culture is mandatory for a late-onset sepsis workup.
• Evaluating CBCs: Do not be fooled by a high WBC count alone. Focus on neutropenia (ANC <1,500/mm³) and an I:T ratio \ge0.2, which are the most clinically significant hematologic indicators of neonatal infection.